The present invention relates to imaging apparatus which employ an invisible form of radiation, such as x-rays, to form an image of an object; and more particularly, to detector arrays that convert the invisible radiation into visible light which then is converted into electrical signals.
A computed tomography system employs an x-ray source to project a cone-shaped beam through the object being imaged, such as a medical patient. Upon exiting the object, the x-rays impinge upon a two-dimensional array of radiation detectors. The intensity of the transmitted radiation which strikes each radiation detector is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The detector array has multiple rows of detectors to acquire x-rays attenuation measurements in a plurality of planar slices through the object and the attenuation measurements from the detectors in a given row produce the transmission profile.
The typical radiation detector comprises a scintillator which is a block of crystalline material the emits visible light upon being struck by x-rays. Thus x-rays striking one surface of the scintillator produce light which is emitted from the opposite surface. A body of semiconductor material is attached to that opposite surface by a transparent epoxy. A two dimensional array of photodiodes, arranged in rows and columns, is formed in the semiconductor body to respond to the light received from the scintillator. A plurality of row electrical conductors connect the photodiodes in each row together and plurality of column electrical conductors connect the photodiodes in each column together. Thus an x-ray attenuation measurement can be acquired from a given photodiode by selecting the row and column electrical conductor to which that photodiode is connected. A transistor switch assembly is used to sequentially access each photodiode in the array and transfer the corresponding electrical signal to a data acquisition system (DAS).
The X-ray source and detector array in a conventional CT system are rotated on a gantry around the object in an imaging plane and so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at a given angle is referred to as a xe2x80x9cviewxe2x80x9d and a xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different angular orientations during one revolution of the x-ray source and detector. The x-ray attenuation data is processed to construct an image that corresponds to a plurality of two dimensional slices taken through the object. The prevailing method for reconstructing an image from that data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d, which are used to control the brightness of a corresponding pixel on an image monitor.
In an unrelated field of technology, optical data communication, microphotonic devices have been developed to route light beams which are modulated with signals carrying data, audio or video. Traditionally, when the signal had to be switched from one optical fiber to another in order to direct the information to the intended recipient location, the optical signal was transformed into an electrical signal which was routed through conventional electrical switching circuits. The electrical signal then was converted back into a modulated light beam for transmission through another optical fiber onward to the recipient location. The conversion between optical and electrical domains slowed the transmission.
In response, microphotonic switching circuits were developed, as described in an article by Peter Fairley entitled xe2x80x9cThe Microphotonics Revolutionxe2x80x9d, Technology Review, July/August 2000, pages 38-64. Microphotonic switching circuits employ tiny electrically operated devices which direct light along a desired path by reflecting or gating the light. For example, a microphotonic switch for routing telecommunication signals from an incoming optical fiber to one of a plurality of outgoing optical fibers utilizes a matrix of microscopic mirrors. Each mirror is electrically tilted independently to switch the light beam between a desired pair of fibers. Thus the microphotonic switches eliminate the need to convert the incoming optical signal into an electrical signal for switching and then reconvert the electrical signal into an optical signal for transmission through the outgoing fiber.
An radiation detector for an imaging apparatus, such as a computed tomography system for example, utilizes a scintillator to convert invisible radiation into light. A light transmission assembly is coupled to the scintillator, thereby defining a plurality of detection sites in the scintillator. An optical conduit leads from the light transmission assembly to data processing circuits. The light transmission assembly has a plurality of microphotonic routing matrices, each one is selectively operable to control the flow of light from one of the detection sites to the optical conduit.
The microphotonic routing matrixes may comprise a plurality of electrically steerable mirrors to reflect the light from the respective detection sites into the optical conduit. In another version, the microphotonic routing matrices comprise a plurality of light gates which are independently operable to transmit the light between the scintillator detection sites and the optical conduit. For example, the light gates may be formed by liquid crystal elements, the light transparency of which is electrically controllable.
In one embodiment, microelectromechanical (MEMS) steerable mirrors is located in a two-dimensional array adjacent to the scintillator. Activation of a given mirror, tilts that device to reflect the light emitted from the respective detection site of the scintillator along a defined path toward a linear array of microelectromechanical steerable mirrors. Each mirror in the linear array receives the reflected light from the MEMS devices in a given row of the two-dimensional array and when tilted directs that light into the optical conduit. By sequentially operating the mirrors in each array, light from every detection site is sent through the optical conduit.